MSGID: 1:317/3 649a660d   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    'Toggle switch' can help quantum computers cut through the noise    
    The novel device could lead to more versatile quantum processors with   
   clearer outputs.    
      
     Date:   
         June 26, 2023   
     Source:   
         National Institute of Standards and Technology (NIST)   
     Summary:   
         What good is a powerful computer if you can't read its output? Or   
         readily reprogram it to do different jobs? People who design quantum   
         computers face these challenges, and a new device may make them   
         easier to solve.   
      
      
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   FULL STORY   
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   What good is a powerful computer if you can't read its output? Or readily   
   reprogram it to do different jobs? People who design quantum computers   
   face these challenges, and a new device may make them easier to solve.   
      
   The device, introduced by a team of scientists at the National Institute   
   of Standards and Technology (NIST), includes two superconducting quantum   
   bits, or qubits, which are a quantum computer's analogue to the logic   
   bits in a classical computer's processing chip. The heart of this new   
   strategy relies on a "toggle switch" device that connects the qubits to   
   a circuit called a "readout resonator" that can read the output of the   
   qubits' calculations.   
      
   This toggle switch can be flipped into different states to adjust   
   the strength of the connections between the qubits and the readout   
   resonator. When toggled off, all three elements are isolated from each   
   other. When the switch is toggled on to connect the two qubits, they can   
   interact and perform calculations. Once the calculations are complete,   
   the toggle switch can connect either of the qubits and the readout   
   resonator to retrieve the results.   
      
   Having a programmable toggle switch goes a long way toward reducing noise,   
   a common problem in quantum computer circuits that makes it difficult   
   for qubits to make calculations and show their results clearly.   
      
   "The goal is to keep the qubits happy so that they can calculate without   
   distractions, while still being able to read them out when we want to,"   
   said Ray Simmonds, a NIST physicist and one of the paper's authors. "This   
   device architecture helps protect the qubits and promises to improve our   
   ability to make the high-fidelity measurements required to build quantum   
   information processors out of qubits."  The team, which also includes   
   scientists from the University of Massachusetts Lowell, the University   
   of Colorado Boulder and Raytheon BBN Technologies, describes its results   
   in a paper published today in Nature Physics.   
      
   Quantum computers, which are still at a nascent stage of development,   
   would harness the bizarre properties of quantum mechanics to do jobs   
   that even our most powerful classical computers find intractable, such   
   as aiding in the development of new drugs by performing sophisticated   
   simulations of chemical interactions.   
      
   However, quantum computer designers still confront many problems. One   
   of these is that quantum circuits are kicked around by external or even   
   internal noise, which arises from defects in the materials used to make   
   the computers. This noise is essentially random behavior that can create   
   errors in qubit calculations.   
      
   Present-day qubits are inherently noisy by themselves, but that's not   
   the only problem. Many quantum computer designs have what is called a   
   static architecture, where each qubit in the processor is physically   
   connected to its neighbors and to its readout resonator. The fabricated   
   wiring that connects qubits together and to their readout can expose   
   them to even more noise.   
      
   Such static architectures have another disadvantage: They cannot be   
   reprogrammed easily. A static architecture's qubits could do a few   
   related jobs, but for the computer to perform a wider range of tasks,   
   it would need to swap in a different processor design with a different   
   qubit organization or layout. (Imagine changing the chip in your laptop   
   every time you needed to use a different piece of software, and then   
   consider that the chip needs to be kept a smidgen above absolute zero,   
   and you get why this might prove inconvenient.)  The team's programmable   
   toggle switch sidesteps both of these problems. First, it prevents circuit   
   noise from creeping into the system through the readout resonator and   
   prevents the qubits from having a conversation with each other when they   
   are supposed to be quiet.   
      
   "This cuts down on a key source of noise in a quantum computer,"   
   Simmonds said.   
      
   Second, the opening and closing of the switches between elements are   
   controlled with a train of microwave pulses sent from a distance, rather   
   than through a static architecture's physical connections. Integrating   
   more of these toggle switches could be the basis of a more easily   
   programmable quantum computer. The microwave pulses can also set the order   
   and sequence of logic operations, meaning a chip built with many of the   
   team's toggle switches could be instructed to perform any number of tasks.   
      
   "This makes the chip programmable," Simmonds said. "Rather than having   
   a completely fixed architecture on the chip, you can make changes via   
   software."  One last benefit is that the toggle switch can also turn   
   on the measurement of both qubits at the same time. This ability to ask   
   both qubits to reveal themselves as a couple is important for tracking   
   down quantum computational errors.   
      
   The qubits in this demonstration, as well as the toggle switch and   
   the readout circuit, were all made of superconducting components that   
   conduct electricity without resistance and must be operated at very cold   
   temperatures. The toggle switch itself is made from a superconducting   
   quantum interference device, or "SQUID," which is very sensitive to   
   magnetic fields passing through its loop.   
      
   Driving a microwave current through a nearby antenna loop can induce   
   interactions between the qubits and the readout resonator when needed.   
      
   At this point, the team has only worked with two qubits and a single   
   readout resonator, but Simmonds said they are preparing a design with   
   three qubits and a readout resonator, and they have plans to add more   
   qubits and resonators as well. Further research could offer insights   
   into how to string many of these devices together, potentially offering   
   a way to construct a powerful quantum computer with enough qubits to   
   solve the kinds of problems that, for now, are insurmountable.   
      
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   ==========================================================================   
   Story Source: Materials provided by   
   National_Institute_of_Standards_and_Technology_(NIST).   
      
   Note: Content may be edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. T. Noh, Z. Xiao, X. Y. Jin, K. Cicak, E. Doucet, J. Aumentado,   
      L. C. G.   
      
         Govia, L. Ranzani, A. Kamal, R. W. Simmonds. Strong parametric   
         dispersive shifts in a statically decoupled two-qubit cavity QED   
         system. Nature Physics, 2023; DOI: 10.1038/s41567-023-02107-2   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/06/230626164157.htm   
      
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